Communication devices such as terminals or wireless devices are also known as e.g. User Equipments (UEs), mobile terminals, wireless terminals and/or mobile stations. Such terminals are enabled to communicate wirelessly in a wireless communication system or a cellular communications network, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
The above terminals or wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals or wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. an “eNB”, an “eNodeB”, “NodeB”, a “B node”, or a Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals or wireless devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
In a wireless communications system, such as a cellular network, a wireless device, e.g. a mobile station, is periodically monitoring the surrounding radio environment, in order for the mobile station to be connected to the most suitable cell, e.g. to the most suitable radio base station serving the cell, or at least to a suitable cell. The selection of the suitable cell may e.g. be based on network decision or by autonomous mobile decision. It should be understood that the expression “the mobile station is connected to a cell” or similar is to be interpreted as the mobile station is connected to a radio network node, e.g. a radio base station or a similar device, that serves or provides radio coverage in a geographical area referred to as the cell. Further, it should be understood that sometimes herein reference is made to the cell and the radio network node serving the cell interchangeably. Thus, sometimes when a reference is made to the cell it should be interpreted as a reference to the radio network node serving the cell.
The initial process to select a cell by searching possible different alternatives is usually referred to as cell selection. The process of periodically monitoring cells and, after an initial cell selection, deciding to move to another cell based on the information acquired through monitoring is usually referred to as a cell reselection procedure.
For cell reselection the process typically involves monitoring different signal strengths from different cells, e.g. from different radio base stations, and ordering them in descending order. That is, the stronger the signal strength from the cell is, the more suitable is the cell to camp on. In case all cells being monitored are weak in signal strength there is typically a minimum allowed signal strength for a cell to be considered as a suitable cell by the mobile station.
By frequently monitoring the surrounding radio environment the mobile station may be ensured to be camping on the cell serving the mobile station with the strongest signal strength, or at least one of the stronger cells, given its current position. In case the mobile station is moving around, the radio environment will change. Hence, to be fully up to date on the surrounding radio environment, the mobile station would have to frequently search for and monitor neighboring cells and to monitor the serving cell, in order to understand the changes over time. This procedure consumes energy, and it will more quickly drain the battery, the more frequently these search procedures are performed. That is a drawback for a mobile station that is dependent on a battery for power.
Although there are mobile stations that benefit from frequent cell-reselection procedures, there are other classes of mobile stations that rather trade a longer battery lifetime for a sub-optimum cell re-selection procedure. At the same time, it is always of interest for any mobile station to be connected to the best cell, e.g. to the cell having the highest signal strength, or at least to a good cell, e.g. to a cell having a high signal strength, to minimize interference levels in the communications network, and hence minimize transmission and/or reception times.
A possible change to the cell re-selection behavior is to allow the mobile station to stay camped on a specific cell as long as the minimum allowed signal strength is fulfilled. In other words, as long as the minimum signal strength is fulfilled, the mobile station would not search for other cells to camp on. However, this would imply that a significant degradation of signal strength of the camped cell would be allowed while still not triggering cell re-selection. If this occurs and better cells are available in the mobile station's current location this would imply increased interference levels in the communications network. Further, it would imply draining of battery during activity by the mobile station since a reduced experienced Signal-to-Interference-plus-Noise Ratio (SINR) would imply a longer ON time for the mobile station both for transmission and reception.
This is illustrated in FIG. 1 wherein it is shown that a mobile station MS has detected, by searching all possible cells, a first cell Cell A, served by a first Radio Network Node (RNN) RNN_A, to be the most suitable cell to camp on when the mobile station MS is located in a first position P1. As the mobile station MS moves to a second position P2, it confirms that cell A still fulfils the criterion on minimum signal strength and hence continues to camp on it.
If the mobile station MS would trigger a cell re-selection, e.g. by searching all possible cells, at any point beyond a point X1 as it moves towards position P2, it would identify a second cell Cell B, served by a second RNN RNN_B, as being the most suitable cell to camp on. This is since the second cell Cell B has a stronger signal strength than the first cell Cell A from the point X1 all the way to the second position P2. However, according to the above described behavior to allow the mobile station to stay camped on a specific cell as long as the minimum allowed signal strength is fulfilled, it is only when the mobile station MS reaches the point X2 it will be triggered to perform a cell re-selection. Thus, it is only at the point X2 the mobile station MS will select the second cell Cell B to be its serving cell, e.g. it is only at the point X2 the mobile station MS will be camping on the second cell Cell B. A drawback is that the mobile station MS continues to camp on the serving cell even if a better cell is available and even if a cell reselection to the better cell would provide better performance.